Kaolin is a naturally occurring hydrated aluminum silicate that is white in color. Kaolin clay as mined, however, is impure, being associated with a variety of contaminants. These contaminants cause the clay to be discolored. The observed discoloration of native kaolin clay is largely due to discoloring mineral contaminants of iron and titanium. Clays vary considerably in their color properties even when produced from mines in the same district or from different sites in the same deposit.
The value of kaolin clays in many applications depends on its color (whiteness) or brightness. A clay is often rejected as unsuitable for commercial use solely on the basis of color even though other physical properties, such as viscosity and particle size distribution, are within desired limits.
One of the largest consumers of clay, particularly kaolinite, is the paper industry. In the manufacture of paper, kaolin is commonly used as a low cost filler or coating pigment to improve the quality of the resulting product. In the absence of a filler, paper has a relatively poor texture due to discontinuities in the fibrous web. Coating clays not only improve the surface characteristics of the paper, but also improve the opacity and brightness of the paper. Usually, the paper industry requires a coating clay with G.E. brightness of 85 to 92.
Ultimate clay brightness is primarily dictated by the starting crude kaolin and the beneficiation to which the kaolin has been subjected. If, in addition to mineral contaminants, organic matter also is present, beneficiation of the clay to a desired brightness value becomes difficult, if not impossible. Minute amounts of organic matter may be sufficient to darken the treated clay appreciably. In addition to any darkening influence, organic matter also may have an extraordinary ability to inhibit or "poison" the leaching action of dithionite, a chemical commonly used to enhance clay brightness. As little as 0.03% organic matter expressed as carbon often may reduce or even eliminate the brightening effect of dithionite leaching. In this regard, large deposits of gray clays are known but such clays have only limited utility since it is not feasible to attain acceptable brightness levels using prior art beneficiation methods.
Various beneficiation methods, including floatation, flocculation, magnetic separation and chemical leaching, have been applied in various combinations by skilled kaolin technologists. Such kaolin beneficiation methods are disclosed, for example, in U.S. Pat. Nos. 3,193,344, 3,798,044, 3,826,365, 3,853,983, 3,961,971, 3,974,067, 4,055,485, 4,097,372, 4,227,920, 4,281,799, 4,419,228, 4,492,628, 4,781,298, 4,851,048, 4,997,550, 5,061,461, and 5,085,707, the disclosures of which are incorporated herein by reference.
It is well known that the brightness of clays usually is increased by fractionation to recover the finer particle fractions. However, this increase is insufficient to render the more discolored clays such as organically contaminated clays, commercially acceptable. In particular, such contaminants are often concentrated in the fine fractions.
It also is known to bleach refined clays with various chemicals to achieve satisfactory brightness levels. Leaching with chemicals such as zinc or sodium dithionite generally results in improved brightness of the beneficiated clay slurries, but typical increases of 2 to 5 brightness points are usually diminished or negated with gray clays as noted above. Brightness of clays can also be significantly improved by use of magnetic separation, froth flotation, and oxidation. However, prior art oxidation methods, including hypochlorite oxidation and ozonation used alone or in conjunction with dithionite leaching and magnetic separation, often do not attain the desired results. Ozonation is carried out on a large scale by all major kaolin producers in the United States. While these companies have developed a high level of technical and industrial practice, the current level of technology does not permit utilization of vast reserves of organically contaminated clays.
Although it is known that calcination of kaolin particulates at 600.degree.-2000.degree. F. oxidizes and destroys organic matter, and often improves brightness significantly, this treatment dehydroxylates the kaolin and interferes with many of the desired clay properties, particularly for coating applications. There currently exist vast reserves of organically contaminated kaolin clays which are discolored to a degree where no commercially practical process currently available to the art sufficiently improves clay brightness values to a level acceptable for using the clay as paper pigment or filler. Current coating clay brightness specifications require a G.E. brightness of about 85 to 92. Thus, a need in the art exists for a beneficialion method which can be used to brighten organically contaminated kaolin clays without dehydroxylating the kaolin, thereby rendering currently unusable reservoirs of clay usable, in particular, for use as a paper pigment or filler.